1. Field of the Invention
The present invention relates to an exposure control apparatus adaptable for a photographing apparatus in a microscope. More particularly, the present invention relates to a shutter speed determining apparatus in which a photo multiplier is used as a photometrical photo-electric element and a proper shutter speed is determined based on the photo-electrically converted signal from the photo multiplier.
2. Description of Prior Art
In the art of still cameras there is already known exposure control apparatus which can automatically determine a proper shutter speed by means of a signal obtained by photoelectric conversion of the light transmitted through an object lens.
Recently, many attempts have been made to develop such automatic exposure control apparatus adaptable also for a photographing apparatus associated with a microscope. However, development of such automatic shutter speed determining apparatus adaptable for a microscope photographing apparatus involves some difficulties. The intensity of light coming from the sample piece in a microscope varies from one sample to another in a very broad range ranging from very dark to fair by bright. To achieve automatic photographing of such objects widely different one from another in brightness, man has to select at first such photo-electric element which has a very wide photometric sensitivity range. Semiconductor photoelectric elements such as photo cells have an advantage of being small in size. However, they have no sensitivity to very dark light such as that coming from a fluorescent sample. Therefore, use of a photo multiplier must be considered. However, this also involves some problems. Those photo multipliers which have a sufficiently broad photometric range to cover from very dark to fair by bright are generally large in size and also expensive. Therefore, they are unsuitable for use in a photographing apparatus of a microscope. Use of a small size photo multiplier has a problem of inability to cover a wide photometric range. To cover a wide photometric range with a small size photo multiplier, it is required to make variable the sensitivity of the photo multiplier. In other words, the sensitivity of such photo multiplier has to be switched over to change the photometric range from one to another thereby covering a wide photometric range as a whole. Hitherto, setting of sensitivity for such small photo multiplier, for instance, in a photometer has been carried out manually. However, when such small size photo multiplier is used in a photographing apparatus for a microscope, it is desirable that the sensitivity setting can be carried out automatically.